1. Field of the Invention
This invention relates to the field of pressure sensitive adhesives and to articles comprising such adhesives.
2. Introduction
Normally tacky, pressure sensitive adhesives (PSAs) are used in the manufacture of a variety of articles such as adhesive tapes and other materials which are intended to be easily attachable to another substrate by the application of pressure alone. Many adhesives preferably have a balance of one or more properties such as tackiness at the temperature of use, adhesion (peel resistance), cohesion (shear resistance), elongation, elasticity, color clarity and color stability, and resistance to sunlight and other ultraviolet and degrading radiation sources. Maintaining the requisite balance of such properties while improving one or more characteristics of such pressure sensitive adhesives is both difficult and unpredictable. Any modification of adhesive compositions which improves one adhesive property may detrimentally affect one or more other desirable properties. For instance, it is difficult to improve an adhesive'shear strength without reducing one or more other desirable properties.
Homopolymers and interpolymers of a variety of monomers are useful as pressure sensitive adhesives when applied to a backing as solutions, dispersions (emulsions) and/or hot melts. However, many PSA applications require shear strength values higher than those that can be provided by otherwise useful polymers. Some applications require shear strength values of at least about 50 minutes and higher, i.e. on the order of 500 to 10,000 minutes (determined by the shear value test described hereinafter). While there are various ways of improving PSA shear strength, many if not all of these detrimentally affect one or more other desirable properties. Polymer molecular weight can be increased to improve shear strength, but this route generally reduces tack and adhesion. Polar monomers, such as polymerizable carboxylic acids, often increase cohesive strength but also may result in low adhesion and tack. Cross-linking monomers, such as the N-methylol amides, or other crosslinking agents are generally expensive and they may reduce tack and adhesion. Their use also may reduce processability and may impair other properties such as clarity, color stability and UV stability.
The suitability of pressure sensitive adhesive compositions is also influenced by the ease of manufacture of both the adhesive and of articles containing the adhesive and by environmental and personnel safety hazards. For instance, PSAs are often applied to a backing as hot melts, polymer solutions or as dispersions of a polymer in an aqueous medium. Such solutions and dispersions must possess properties which facilitate their use in the manufacture of PSA-containing articles. Thus, the melt, solution or dispersion, as well as the polymer per se, must adequately wet the backing to assure adequate adhesive distribution, coverage and bonding to the backing.
The chemical composition of the PSA polymer carriers (when used) is also significant for several reasons. Solvents other than water are becoming more and more undesirable due to solvent expense and the cost and hazards involved in controlling solvent vapors. Yet such solvents are often necessary for adequate distribution and handling of polymers that cannot be employed effectively in water-based systems. Thus, water-based polymer latexes are much preferred in the adhesive manufacturing industry provided that the necessary physical and chemical properties of the finished article can be achieved. However, substitution of water-based latexes or hot melts for solvent-based polymers may reduce one or more physical properties.
N-methylol amide functional groups and other crosslinking monomers and agents are known to improve adhesive performance in several respects. However, such polymers release formaldehyde upon curing or can result in the presence of potentially toxic residues in the finished article. In particular, N-methylol amide-containing polymers release formaldehyde when cured, and they can result in formaldehyde residues in the finished product. Such formaldehyde release and residues are often undesirable since formaldehyde is coming under ever increasing scrutiny in both the workplace and home. It is particularly undesirable in medical and personal contact applications, such as adhesive bandages, and the state and federal Occupational Health and Safety Administrations (OSHA) have set stringent formaldehyde exposure limits for industrial workers.
Various rheological properties of water-base latexes are particularly important with regard to the suitability of such latexes for adhesive article manufacture. Latex particle size and particle size distribution can significantly influence latex physical properties which affect application of the latex to a backing. Similarly, latex viscosity can limit latex utility in adhesive article manufacture due to the influence of viscosity on adhesive distribution, filler loading (of the latex) and wetting of the adhesive article backing.
Yet further demands are placed on the chemical composition and physical properties of low temperature pressure sensitive adhesives, i.e., adhesives intended for use at relatively low temperatures. Often, PSAs which have adequate cohesive and adhesive strength at low temperatures are so "gummy" at ambient conditions that they complicate both adhesive handling at ambient temperatures and the manufacture of adhesive-containing articles. Such gummy adhesives also tend to "creep" and to "bleedthrough" labels and other backings.
Thus, it can be seen that the physical and chemical properties desired in adhesive compositions and articles, and in the polymer solutions, dispersions and melts employed in the manufacture of adhesive articles, place various, sometimes conflicting, demands on polymer composition and on the polymer carrier, i.e, solvent or water, if used. It is desirable to obtain a polymer system, preferably a water-base or hot melt system, which possesses a balance of properties suitable for the manufacture of pressure sensitive adhesives and PSA-containing articles.